Method for potting and encapsulating electronic circuits

ABSTRACT

A method for potting and encapsulating electronic circuits by providing a homogeneous mixture of uncured thermosetting resin and curing agent in a flowable powder form with a particle size in the range of approximately 60 mesh to 200 mesh and curable in a range of approximately 125° F. to 212° F., and hollow microspheres having a diameter in the range of approximately 5 microns to 300 microns, intermixing the resin-curing agent and microspheres in a ratio in the range of approximately 35 percent to 45 percent resin-curing agent to microspheres by weight, enclosing the electronic circuit with a form, pouring the homogeneous mixture in the form to surround one electronic circuit, and heating the mixture to the required temperature for curing.

This is a division of application Ser. No. 890,434 filed Mar. 27, 1978,now abandoned.

BACKGROUND OF THE INVENTION

The present invention pertains to material for potting and encapsulatingelectronic circuits utilized in high acceleration and high thermalcycling environments. In high acceleration environments, it is essentialto utilize a material which is light-weight and strong to preventmovement of any of the electronic parts. In the high thermal cyclingenvironments it is essential that the material has a low coefficient oflinear expansion, relatively low modulus of elasticity (ratio of unitstress to unit deformation), high compression strength and low embedmentstress. Other characteristics which must be considered includerepairability, cure stresses and cure temperatures, flow of material,outgassing properties, etc.

In the prior art, it is well known to utilize solid resins and hollowglass microspheres for flotation purposes in water craft and the like.However, these solid resins must first be thoroughly mixed with a curingagent and then the microspheres must be intermixed with the catalyzedresin before it cures. It is extremely difficult to completely mix theresin and curing agent and then to mix the hollow glass spheres into thecatalyzed resin so that each particle of resin is catalyzed by aparticle of curing agent and each glass sphere is affixed to anotherglass sphere by a particle of catalyzed resin therebetween. If theentire mixture is not uniform and substantially homogeneous there willbe a great variation in the physical characteristics thereof, not onlybetween different batches of mixture but within a single batch.

These materials are not suitable for potting and encapsulatingelectronic circuits because the physical characteristics are unsuitable.For example, most known materials, of this type have a cure temperaturefar in excess of that acceptable with electronic circuits, i.e., theelectronic circuits will be practically ruined by the time the pottingmaterial is cured or the cure time is too long to be economicallyfeasible.

SUMMARY OF THE INVENTION

In the present invention, potting and encapsulating material forelectronic circuits utilized in high acceleration and high thermalcycling environments is prepared by intermixing a compound of uncuredthermosetting resin and curing agent in a flowable powder form with aparticle size in the range of approximately 60 mesh to 200 mesh andcurable in a range of approximately 125° F. to 212° F. with hollowmicrospheres having a diameter in the range of approximately 5 micronsto 300 microns, said mixture being in a ratio in the range ofapproximately 35 percent to 45 percent resin-curing agent tomicrospheres by weight, the exact ratio being determined by the desiredfinal properties of the material, i.e., controlled compressive andcrushing strength (imbedment stress, modulus of elasticity, sheerstrength, compression strength and coefficient of linear expansion),controlled outgassing, controlled thermal expansion, controlledrepairability and controlled thermal conductivity.

It is an object of the present invention to provide potting andencapsulating material for electronic circuits utilized in highacceleration and high thermal cycling environments, which material hasphysical properties which vary in accordance with the ratio of theresin-curing agent and microspheres utilized.

It is a further object of the present invention to provide a method ofproducing potting and encapsulating material for electronic circuitsutilized in high acceleration and high thermal cycling environments.

These and other objects of this invention will become apparent to thoseskilled in the art upon consideration of the accompanying specificationand claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In high G, or high acceleration, environments, such as shock andvibration, component parts of an electronic circuit move relative toeach other causing circuit performance variations and failures of solderjoints. The traditional approach to immobilizing these components hasbeen to secure them with adhesive bonding or staking, or encapsulatingthem in resin or foam. However, a problem comes about when thermalcycling occurs such as is induced upon turning devices on and off.Expansion differentials require some form of stress relief in themounting of the electronic component to allow the flexibility needed toaccommodate the expansion differentials of the material. When componentsare secured for the dynamic environments of shock and vibration, thisrigid securing causes problems in thermal cycling. When parts are stressrelieved for thermal cycling, the less rigid mounting causes problems inshock and vibration. The present invention has been developed to allowsecuring of components for high G level dynamic environments whileallowing high numbers of thermal cycles. In addition, the new materialallows ease of repair, low volatiles for use in "clean" space programsand substantially no pressure exerted on components in the curedcondition. These properties make the material unique so that it can beused on high reliability and space programs. It is equally applicable toairborne and ground equipment and is especially useful on printed wiringassemblies.

To produce the potting and encapsulating material for electroniccircuits, the first step is to select an uncured compound ofthermosetting resin and curing agent in a flowable powder form. Thethermosetting resin must be a material which is curable at a low enoughtemperature so that the curing does not damage the electroniccomponents. Generally, if the resin is curable in a range ofapproximately 125° F. to 212° F. and the curing periods are relativelyshort, the electronic circuits will not be damaged. The final pottingand encapsulating material must be flowable between and around thevarious electronic components and therefore must be in a flowable powderform and it has been found that a satisfactory particle size isgenerally in the range of 60 mesh to 200 mesh. For example, thethermosetting resin diglycidyl ether of bis phenol A and the curingagent 2-phenyl imidazole in a solid state may be processed, for examplein a ball mill or the like, to produce a powder of the desired particlesize. The processed thermosetting resin and curing agent is sifted in ascreen with the desired mesh size to remove any oversized particles. Byprocessing the resin and curing agent in this fashion the two arethoroughly intermixed to ensure proper catalyzing of each resinparticle. The particular resin specified above can be purchased underthe trademark "EPON 1001" of the Shell Chemical Company.

A supply of hollow microspheres may be obtained commercially from avariety of sources and, in general, these microspheres will be formedfrom a glass such as, silica (approximately 95 percent SiO₂), or from athermosetting resin such as phenolic. In general, to ensure the correctflowability and the other physical properties of the potting andencapsulating material, the hollow microspheres should have a diameterin the range of approximately 5 microns to 300 microns and the wallthickness should be in the range of approximately 1 micron to 6 microns.The microspheres purchased commercially generally have a wide sizerange, because of manufacturing techniques. For example in the presentembodiment microspheres having a particle size ranging fromapproximately 30 microns to 125 microns were utilized with a wallthickness of about 2 microns.

The powdered resin and curing agent are then mixed with the microspheresin the following manner. A desired quantity of microspheres is placed ina closed container and an amount of resin-curing agent in the range ofapproximately 35 percent to 45 percent resin-curing agent tomicrospheres by weight, is also placed in the closed container. Thecontainer is then closed to prevent the loss of material and shookvigorously for 2 to 3 minutes. Occasionally the container may berepositioned and the shaking mode may be varied to assure a thoroughmixing action. This homogeneous mixture ensures that the previouslycatalyzed resin coats the adjacent microspheres and forms a type ofmaterial called foam. In general, it is believed that the spheres fuseat the tangent points to give a very cohesive foam material when cured.The potting and encapsulating material, mixed and in the powder formdescribed above, may be stored by promptly placing it in a storage areaat a temperature of approximately 40° F. plus or minus 7° F.

The potting and encapsulating material described above, is utilized bypartially surrounding an electronic circuit with an encapsulating form,permanent shield, or the like so that the potting and encapsulatingmaterial may be contained in surrounding relationship to the electroniccircuit. The form is then filled with the potting and encapsulatingmaterial which, because of its high flowability, completely surroundsthe components of the electronic circuit. To assure void free pottingthe form or mold should be gently tapped or vibrated to promote flow andsettling of the material. If the electronic circuit is to beencapsulated and the form or mold removed thereafter it is best to linethe mold or form surfaces with Teflon tape or Teflon film so that themold will release upon completion of the encapsulation. Once the mold orform is completely filled the entire structure will be placed in an ovenand the potting and encapsulating material is cured at the requiredtemperature. In general, the maximum allowable curing temperature is100° C. for a two hour period, 82° C. for a three hour period or 74° C.for a four hour period. Because of the possible damage to the electroniccomponents in the circuit, it is preferable to use the lowest possiblecuring temperature for the least amount of time.

It is especially crucial that there is very little or no shrinkage ofthe potting and encapsulating material during the cure step. It has beenfound, for example, that the greater the ratio of resin-curing agent tomicrospheres by weight of the potting and encapsulating material, thegreater will be the shrinkage upon curing the material. When the ratioof the resin-curing agent to microspheres exceeds approximately 45percent the shrinkage of the material upon curing becomes excessive andcan cause damage to the electronic circuits being potted orencapsulated. Shrinkage of the plastic material may produce pressure oncomponents which changes their electrical characteristics, or it maybreak solder joints and the like.

A second characteristic of the potting and encapsulating material, whichis closely aligned with the shrinkage characteristic, is commonlyreferred to as embedment stress. Embedment stress is the amount ofpressure produced on components in the electronic circuit due totemperature changes of the potting and encapsulating material. Again, ifthe embedment stress is excessive it will cause a change in electroniccharacteristics of the components and/or breakage of connections andsolder joints. Embedment stress can be measured by embedding a pressuretransducer in the plastic material and cycling it through the upper andlower temperature variations. For example, acceptable embedment stressesare under 400 pounds per square inch at a minus 30° F. and under 10pounds per square inch at the cure temperature, generally 90° F. to 212°F. It has been found through extensive experimentation that theembedment stress can be substantially reduced by controlling the ratioof the resin-curing agent to microspheres in the potting andencapsulating material. In general, if the ratio of resin-catalyst tomicrospheres is maintained in the range of approximately 35 percent to45 percent by weight, the embedment stress will be within acceptablelimits.

Because the potting and encapsulating material is being utilized tomaintain electronic components in fixed positions under highacceleration, or G, environments, it is essential that the compressivestrength of the potting and encapsulating material is withinpredetermined limits. As the ratio of resin-catalyst to microspheres isreduced the ultimate compressive strength is also reduced. Thecompressive strength is determined by applying a pressure, measured inpounds per square inch, and measuring the amount of deflection of thepotting and encapsulating material. In general, the ultimate stress ofthe material should be no less than approximately 100 psi. Throughextensive experimentation, it has been found that a 35 percent ratio ofresin-curing agent to microspheres will withstand 112 psi ultimatestress, while a ratio of 45 percent resin-catalyst to microspheres willwithstand a compressive ultimate stress of 150 psi. In addition, acharacteristic commonly referred to as the modulus of elasticity incompression is generally related to the compression characteristic. Themodulus of elasticity is a measure of the pounds per square inch ofstress produced by deflection of the potting and encapsulating materialunder compression. It is extremely important that the stress produced bythe material must be very low (generally under 5 psi) for deflections inthe range of 0 to 0.001 inches per inch of material, or a modulus ofelasticity of 5000 psi.

In addition to the above properties dealing with the physical strengthof the potting and encapsulating material, it is essential that theoutgassing of the material is below predetermined standards if thematerial is to be used in high reliability and missile space programs.Outgassing is a measure of the amount of volatile condensable materialswhich are emitted by the potting and encapsulating material aftercuring. The percent of weight loss of the material because of theemission of the volatile materials is also an indication of theoutgassing characteristic. The outgassing characteristic is measured ina vacuum, generally less than 10⁻⁵ Torr. The percent volatilecondensable materials which is acceptable is below approximately 0.1percent and the percent weight loss which is acceptable is belowapproximately 1.0 percent.

EXAMPLE

A flowable powdered potting and encapsulating material was producedusing 40 percent by weight of an uncured thermosetting resin, diglycidylether of bis phenol A, with the curing agent 2-phenyl imidazole having aparticle size such that it passed through a 150 mesh screen and hollowglass microspheres (over 95 percent SiO₂) having a diameter between 30and 125 microns with a wall thickness of approximately 2 microns. Thepowdered resin-curing agent and microspheres were placed in a closedcontainer and shook vigorously for two to three minutes with thecontainer being occasionally repositioned to vary the shaking mode andassure a thorough mixing action. An electric circuit was potted with theabove material by completely filling a form partially surrounding thecircuit and curing the material for two hours at 100° C. The curedmaterial produced no measurable shrinkage upon curing. Further, thematerial has an ultimate compressive strength of 134 psi. And themodulus of elasticity was such that 0.001 inches per inch of deflectionproduced less than 2 psi of stress (modulus of elasticity of 2000 psi).Upon temperature cycling the potted structure, well under 400 psi ofembedment stress was exhibited at minus 30° F. and under 10 psi ofembedment stress was exhibited between 90° F. and 212° F. Also, in theoutgassing tests 0.02 percent volatile condensable materials wereproduced and 0.8 percent weight loss was exhibited.

A sample of the above potting and encapsulating material was utilized inwhich 35 percent by weight of resin-curing agent was included. Theultimate compressive strength dropped to 112 psi. The embedment stresswas slightly reduced.

A third sample of the potting and encapsulating material was utilizedwherein 45 percent by weight of resin-curing agent was included. In thissample the ultimate compressive strength rose to approximately 150 psiwith the embedment stress increasing over the first sample somewhat.Because of the ultimate compressive strength requirements, it wasdetermined that a range of approximately 35 to 45 percent by weight ofresin-curing agent to microspheres is required to obtain the desiredphysical characteristics.

While we have shown and described specific embodiments of thisinvention, further modifications and improvements will occur to thoseskilled in the art. We desire it to be understood, therefore, that thisinvention is not limited to the particular form shown and we intend inthe appended claims to cover all modifications which do not depart fromthe spirit and scope of this invention.

What is claimed is:
 1. A method of potting an electronic circuitutilized in high acceleration and high thermal cycling environmentscomprising the steps of:(a) selecting an uncured compound consistingessentially of thermosetting resin and curing agent in a flowable powderform with a particle size in the range of approximately 60 mesh to 200mesh, curable in a temperature range of approximately 125° F. to 212° F.at approximately ambient pressure and which produces, in a vacuum ofless than 10⁻⁵ Torr, outgassing of a maximum of 0.1 percent volatilecondensible materials and a maximum of 1.0 percent weight loss; (b)selecting a quantity of hollow microspheres having a diameter in therange of approximately 5 microns to 300 microns; (c) mixing saidcompound and said microspheres into a substantially homogeneous mixtureconsisting essentially of said compound and said microspheres in apredetermined ratio in the range of approximately 35 percent to 45percent resin-curing agent to microspheres by weight; (d) partiallyenclosing the electronic circuit with a form to limit the flow ofpotting material; (e) pouring the homogeneous mixture in the form tosurround the electronic circuit with mixture; and (f) heating themixture to the required temperature for curing in a temperature range ofapproximately 125° to 212° F. at approximately ambient pressure.
 2. Amethod of potting an electronic circuit utilized in high accelerationand high thermal cycling environments comprising the steps of:(a)providing an uncured compound consisting essentially of diglycidyleither of bis phenol A and 2-phenyl imidazole in a flowable powder formwith a particle size in the range of approximately 60 mesh and 200 meshand curable in a temperature range of approximately 125° F. to 212° F.at approximately ambient pressure; (b) selecting a quantity of hollowmicrospheres having a diameter in the range of approximately 5 micronsto 300 microns; (c) mixing said compound and said microspheres into asubstantially homogeneous mixture consisting essentially of saidcompound and said microspheres in a predetermined ratio in the range ofapproximately 35 percent to 45 percent resin-curing agent tomicrospheres by weight; (d) partially enclosing the electronic circuitwith a form to limit the flow of potting material; (e) pouring thehomogeneous mixture in the form to surround the electronic circuit withmixture; and (f) heating the mixture to the required temperature forcuring in a temperature range of approximately 125° to 212° F. atapproximately ambient pressure.